The present invention generally relates to a detection apparatus and method for detecting an amount of a physical variable, and more particularly to a detection apparatus and method for detecting an amount of a physical variable to provide a signal corresponding to an amount which can be digitally processed.
As a prior art example of a detection circuit adapted to detect a change in a physical amount, it is described in Japanese Patent Public Disclosure (Laid-Open or Kokai) No. 63-108257 issued in 1988, for example. FIG. 1 is a block diagram illustrating the prior art detection circuit described in the No. 63-108257. The detection circuit, which is intended to detect humidity as a physical amount, comprises an oscillation unit 51 having a humidity sensor 54, a F-V converter 52, and a logarithmic amplifier 53. The humidity sensor 54 is designed to vary its resistance in accordance with variations in ambient humidity. Then, the resistance variation causes the oscillation unit 51 to change its oscillation frequency. An output signal of the oscillation unit 51 is next input to the F-V converter 52, where the frequency of the signal is converted to a direct current voltage. The direct current voltage signal output from the F-V converter circuit 52 is next inputted to the logarithmic amplifier 53, where the direct current voltage is logarithmically amplified. In this way, this detection circuit can reveal the ambient humidity on the bases of the output voltage value from the logarithmic amplifier 53.
Another prior detection circuit is described in Japanese Patent Publication (Kokoku) No. 2-22338. This detection circuit also detects a change in humidity as is the case of the above-mentioned No. 63-108257. Although not shown here, the detection circuit comprises an integrator for detection including a humidity sensor as a capacitance which changes its value in response to humidity, and a reference integrator for comparison which does not change the time constant. In an operation of the detection circuit, the same pulse signal is input to both the integrators, and the difference between signals output from the respective integrators is delivered from a differential amplifier, and a maximum value of the difference is further output from a peak hold circuit as a direct current voltage signal. Thus, the detection circuit can provide the ambient humidity based on the output voltage of the peak hold circuit.
Another prior art example of a detection circuit is described in Japanese Patent Public Disclosure (Kokai) No. 63-27720. FIG. 2 is a circuit diagram illustrating the detection circuit which is for weight detection described in No. 63-27720.
In recent years, integrated circuit technologies have advanced and digital signal processing requiring complicated product/sum operations can be readily performed by using a processor dedicated to signal processing or the like. Since such digital signal processing enables time division processing or the like using a software control, a large amount of complicated signals can be processed to precisely reveal a variety of information while avoiding a system from requiring a larger size and an increased cost.
Each of the detection circuits described in the above-mentioned Japanese Patent Public Disclosure Nos. 63-108257 and 2-22338, however, outputs, from its output terminal, an analog signal or voltage which has magnitude depending on the physical amount. It is therefore necessary to convert the output voltage into a digital signal by an additional A/D converter in order to digitally process it. For this reason, if the digital processing apparatus such as a microcomputer is introduced to process the output of the detection circuit, a more complicated configuration such as an A/D converter is required therebetween, thus causing a problem of an increased size and an increased cost of the entire system. Particularly, when parallel real time processing is required for a large number of signals output from such detection circuits, it is necessary to parallelly provide a number of A/D converters equal to the number of signals from the detection circuits, making the above problem more prominent.
On the other hand, as illustrated in FIG. 2, the detection circuit described in Japanese Patent Public Disclosure No. 63-27720 comprises an oscillation unit 42 including operational amplifiers 42a, 42b and a sensor 41 which changes the capacitance in accordance with a weight applied thereto. An oscillation frequency of the oscillation unit 42 changes in response to a change in the capacitance of the sensor 41. A variable resistor 43 is also provided in the oscillation unit 42 for adjusting a basis of the oscillation frequency.
An output signal of the oscillation circuit 42 is input to an amplifier 46 including a transistor 46a. The amplifier 46 amplifies the output signal of the oscillation unit 42 so as to have an enough amplitude that a counter 47 in a microcomputer 45 can count the number of waves of the oscillation signal. Thus, the counter 47 counts the number of the waves in the amplified signal during a predetermined time period, and outputs a count value to a processing unit 48 in the microcomputer 45. A voltage setting circuit 44 in turn sets a predetermined direct current voltage. This direct current voltage is input to an A/D converter 49 in the microcomputer 45 where it is converted into a digital signal, and then output to the processing unit 48. The processing unit 48 calculates the capacitance of the sensor 41 from the count value, using the digital value input from the A/D converter 49 as a conversion coefficient.
In the detection circuit shown in FIG. 2, the oscillation unit 42 converts a change in the capacitance of the sensor 41 into a change in frequency. Then, the counter 47 counts the number of the waves in the frequency signal from the oscillation unit 42 so that the capacitance change of the sensor 41 can be revealed as a digital signal.
However, in the detection circuit shown in FIG. 2, any parasitic capacitance is inevitably formed at an input terminal of the operational amplifier 42a or the like. Therefore, when a sensor 41 having an extremely small capacitance must be used, a change in the capacitance of the sensor 41 does not induce an apparent change in frequency of the output signal due to the influence of the parasitic capacitance. Particularly, in the approach which counts the number of waves in the signal output from the oscillation unit 42 in a predetermined period to reveal a change in the capacitance of the sensor 41, only a change in frequency exceeding a certain level eventually represents a change in the number of the waves. Therefore, it causes a problem in that a change in the capacitance of the sensor 41 is difficult to be captured when a change in the oscillating frequency does not reach the level. It is contemplated to make the oscillating frequency of the oscillation unit 42 higher and use a very high speed counter 47 in order to solve such a problem as above. However, the solution would result in a more complicated circuit configuration and therefore a very expensive apparatus. Furthermore, the parasitic capacitance as mentioned becomes significantly larger when the operational amplifier 42a of the oscillation unit 42 and the sensor 41 are formed on separate chips. Consequently, such an increased parasitic capacitance would make it difficult to produce stable oscillation in the oscillation unit 42.
Further, in the detection circuit shown in FIG. 2, a change in the counted number of the waves must be converted into a change in a capacitance through digital processing in the processing unit 48. However, the oscillation frequency of the oscillation unit 42 as mentioned above hardly exhibits a simple proportional relationship with the capacitance value of the sensor 41. In other words, complicated operations such as square and inversion operations must be performed at high speed in the processing unit 48 in order to reveal a change in the capacitance of the sensor 41 in real time. Therefore, unless a particularly expensive and high performance microcomputer is employed, most of the capabilities of the processing unit 48 would be used up by such operations.
The present invention has been made to solve the problems inherent to the prior arts. Accordingly, an object of the present invention is to provide detection apparatus and a method using an oscillation unit, an output frequency of which varies reliably depending on an impedance such as a capacitance of a sensor.
Another object of the present invention is to provide detection apparatus and method using an oscillation unit an output frequency of which varies substantially proportional to an impedance such as capacitance of a sensor.
Further object of the present invention is to provide detection apparatus and method which are capable of constantly capturing a change in a capacitance of a sensor without fail in a simple configuration, irrespective of values of the sensor capacitance and parasitic capacitances.
A still further object of the present invention is to provide a detection apparatus and method using an oscillation unit an output frequency of which varies depending on a capacitance of a sensor in which the oscillation unit can provide a square wave in order that a variation in amplitude of the oscillation output does not affect the detection of the capacitance value.
In order to achieve the purposes of the present invention, an apparatus for detecting a sensor impedance which varies in response to a sensed physical amount of at least one sensor, according to the present invention, comprises an impedance-frequency conversion unit for converting the sensor impedance to an oscillation signal a frequency of which corresponds to the sensor impedance and a counter for counting the number of the waves of the oscillation signal in a predetermined time period to output a count value, wherein the impedance-frequency conversion unit comprises an oscillator including the sensor impedance, for generating a square wave signal as the oscillation signal.
In the apparatus mentioned above, the oscillator is preferably a Wien bridge oscillator including an amplifier having a variable gain and a positive feedback circuit of the amplifier, wherein the positive feedback circuit includes a resistor or capacitor as the sensor impedance, and a product of a gain of the amplifier and a positive feedback ratio of the feedback circuit is chosen to be more than or equal to one. Further, it is preferable that the sensor impedance has one end connected to a reference voltage.
The apparatus mentioned above can further comprise additional sensors and counters, wherein the impedance-frequency conversion unit further comprises additional oscillators including the additional sensor impedances respectively, for generating square wave signals as additional oscillation signals to the respective additional counters, frequencies of the additional oscillation signals respectively corresponding to the additional sensor impedances. In the apparatus, the sensors are preferably constituted as a resonator array.
In another aspect of the present invention, there is provided an apparatus for detecting a sensor impedance which varies depending on a sensed physical amount of at least one sensor, the apparatus comprising an impedance-frequency conversion unit for converting the sensor impedance to an oscillation signal a frequency of which corresponds to the sensor impedance and a counter for counting the number of waves of the oscillation signal in a predetermined time period to output a count value, wherein the impedance-frequency conversion unit comprises an impedance-voltage converter for providing an output voltage corresponding to the sensor impedance and an oscillator including a variable impedance element, an impedance of which varies depending on the output voltage of the impedance-voltage converter, for generating the oscillation signal. In addition, a frequency of the oscillation signal depends on a varied impedance of the element.
The apparatus of the second aspect preferably includes a voltage adding unit positioned between the impedance-voltage conversion unit and the oscillator, for adding a predetermined DC voltage to the output voltage of the impedance-voltage converter. The added voltage is provided to the variable impedance element of the oscillator.
In the apparatus of the second aspect, it is preferable that the variable impedance element of the oscillator is formed by a drain-source resistance of a first MOS FET which is variable by a voltage applied to a gate thereof, and the voltage adding unit comprises a second MOS FET having a gate which connected to receive the output voltage corresponding to the sensor impedance and a drain connected to a variable load resistor to provide an added voltage to the gate of the first MOS FET.
In the apparatus of the second aspect, it is preferable for the sensor impedance to be a capacitor and the impedance-voltage converter comprises (a) a first operational amplifier having an inverting input terminal connected to receive an input voltage through a resistor from a variable voltage generator and connected to its output terminal through a resistor and a first switch connected in parallel to each other, and a non-inverting input connected to receive the input voltage through the sensor impedance and a reference voltage through a switch, wherein the input voltage is variable during the switch is turned off, (b) a second operational amplifier having an inverting input connected to receive the input voltage through a resistor from the variable voltage generator and connected to its output through a resistor and a second switch connected in parallel to each other, and a non-inverting input connected to a reference voltage terminal, and (c) a third operational amplifier having a non-inverting input connected to receive an output voltage from the first operational amplifier, and an inverting input connected to receive an output voltage from the second operational amplifier and connected to an output through a variable resistor and a third switch connected in parallel to each other, wherein the output is connected to the gate of the second MOS FET, and the first through third switches are turned on to reset the impedance-voltage conversion unit and turned off before starting a measurement of the impedance. The first through third switches are turned on during a reset or initialization cycle and turned off before starting a measurement cycle.
A method of detecting a sensor capacitance which varies in response to a sensed physical amount of a sensor, according to the present invention comprises the steps of (a) converting the sensor capacitance to a voltage corresponding thereto, (b) varying an impedance of an element in response to the converted voltage, (c) generating a frequency signal from an oscillator, which varies in response to the impedance of the element, (d) counting the number of waves of the frequency signal from the oscillator in a predetermined time period, whereby the sensor capacitance is converted to the oscillation frequency signal which is a digital form.